Effect of molar mass distribution (MMD) and composition distribution (CD) on crystallization behavior of linear low-density polyethylene materials at moderate and high supercooling was studied using differential scanning calorimetry, hot-stage polarized light microscopy, small-angle light scattering, and chip nanocalorimetry methods. A set of uni- and bimodal materials having small variation in average molar mass, density, and melt flow rate, but large differences in MMD and CD, was investigated. The results indicate a clear effect of structural heterogeneity on morphology and crystallization behavior of the materials. Broader MMD and CD increased in average radius of superstructures, melting, crystallization temperatures, and isothermal crystallization rate at different supercoolings. Origin of such behavior is discussed. (C) 2008 Wiley Periodicals, Inc.